The present invention relates to a structure and process for injection molding optical and compact disks.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal materials such as iron, steel, stainless steel, aluminum, or brass because these materials have high thermal conductivity and thus permit the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. A drawback to rapid cooling in these molds is that the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates a rough surface (instead of a high quality optical surface) which can impact disc performance. The quick solidification of the melt combined with variable radial flowability of the materials makes the uniform melt flow and uniform surface replication required for an optical disk difficult to achieve. Non-uniform flow and surface imperfections can result in areas on an optical disk with high bit errors.
In the injection molding of compact discs, for audio, video, or computer data storage and retrieval applications, heat transfer through the mold thus has a strong effect on molding time and disc attributes such as birefringence, flatness, and accuracy of feature replication. For a process to be economical, a careful balance must be maintained between low cycle times and the process parameters needed to meet the quality requirements.
A method for affecting heat transfer and improving the cycle time during injection molding by incorporating insulation into the mold has been described in commonly assigned Kim et al., U.S. Pat. No. 5,458,818. In Kim et al., a multilayer mold is used in which a metal core has an insulating layer bonded thereto for slowing the initial cooling of the resin during the molding operation. The insulating layer comprises material having both low thermal diffusivity and conductivity, thus slowing the cooling of the molded resin, and good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. One or more skin layers of hard material, typically metal, can be bonded to the insulating layer.
Another method for affecting heat transfer is described in Nakamura et al., Japanese Unexamined Patent Application Disclosure Bulletin No. 88-71325. In Nakamura et al., a layer of synthetic resin is formed on a stamper by coating or lamination before the stamper is placed on the core molding surface of the metal mold.
The use of an insulating layer is desirable so as to cause a minimal change in the size and shape of the molding tool and equipment. For compact discs, stringent requirements of optical clarity, surface morphology, and replication of surface features of submicron dimensions obviate the use of common insulating materials, which do not provide a smooth enough surface, are not stable for long periods at the mold temperature, or cannot withstand the repeated application of high pressure during the molding process.
For a sheet or film to be useful for managing heat transfer for a mold it must have a very smooth surface (&lt;0.1 .mu.m surface roughness) over a large area so that it will not introduce feature replication errors or surface imperfections into the manufactured disk. It is also preferred that the surface be compliant to attenuate minor imperfections in the molding tool while maintaining mechanical and dimensional integrity during the molding process.